Lessons learned using an MRI-only workflow during high-dose-rate brachytherapy for prostate cancer.

PURPOSE We report clinical observations of a technique using an MRI-only workflow for catheter insertion and treatment planning in patients receiving standard-care high-dose-rate brachytherapy before external beam radiotherapy for prostate cancer. METHODS AND MATERIALS Forty patients with intermediate or high-risk prostate cancer were enrolled on a prospective clinical trial approved by our institution's research ethics board. Multiparametric MRI with stereotactic navigation was used to guide insertion of brachytherapy catheters, followed by MRI-based treatment planning. RESULTS Sixty-two implants were performed. Median catheter insertion + imaging time was 100 minutes, and overall anesthesia time was 4.0 hours (range, 2.1-6.9 hours). MRI at the time of brachytherapy restaged 14 patients (35%) who were found to have a higher stage of disease. In 6 patients, this translated in directed insertion of brachytherapy catheters outside the prostate boundary (extracapsular disease [n = 2] or seminal vesicle invasion [n = 4]). Most patients (80%) had gross tumor visible on MRI, which influenced catheter insertion and treatment planning. MRI depicted postimplant anatomic boundaries clearly, with the exception of the apical prostate which was blurred by trauma after catheter insertion. Conventional dose-planning objectives for the rectum (V75 < 1.0 cc) were difficult to achieve, but toxicities were low (acute grade ≥ 2 genitourinary = 20%, late grade ≥ 2 genitourinary = 15%, and late grade ≥ 2 gastrointestinal = 7%). Urethral trauma visualized on MRI led to two transient Grade 3 events. CONCLUSIONS Despite a standard-care approach, MRI acquired throughout the procedure altered catheter insertion and dose-planning strategies. An MRI-only workflow is feasible but must be streamlined for broader acceptance.